Although bats are one of the most diverse groups of mammals today, they are one of the least
common groups in the fossil record. Bats have small, light skeletons that do
not preserve well and we have little information on the early evolution of this group.
The earliest fossil bat (shown here) is a remarkably well preserved animal from early Eocene rocks
in the Green River formation of Wyoming. Given the name Icaronycteris, it comes
from a species that is clearly microchiropteran6.
The oldest known megachiropteran, Archaeopteroptus transiens, is Oligocene
(38-23 mya) in age. It and a Miocene (23-25 mya) fossil from Africa make up the entire
known fossil record of megachiropterans.

Icaronycteris index

When did bats evolve?
The inability to link bats to any other mammalian group in itself suggests a very early
origin. Some fossilised eggs of noctuid moths, with the ablity to detect echolocation
calls of bats and trigger escape responses , have recently been discovered dating back
to about 75 MyBP5 implying that the bats themselves arose substantially earlier,
about 80 to 100 m.y.a.6. If so, they would
have shared their world with dinosaurs, watched their extinction at the end of the
cretaceous and remained, relatively unchanged, to this day.

Why did bats evolve?
At the time bats are thought to have been evolving, the flowering plants were in the first
stages of their massive diversification. By the end of the Cretaceous, the insects
supported by these plants were abundant, and insectivorous mammals were becoming well
established4. But so were the predators of small mammals, posing a serious threat during
daylight hours. For these reasons, it is presumed early species of bat were nocturnal,
evolving from small, arboreal mammals16. From here they literally launched themselves
into flight, becoming the highly successful aerial hunters we see today.

Over thousands of years of jumping around after insects, from tree to tree, the ancestors
of bats
first evolved gliding membranes similar to those of colugos
(order Dermoptera)
. Less energy is expended in gliding from tree to tree than running down the trunk,
across
the ground and back up the next tree. And if an animal doesn't come down to ground, it
doesn't have to face terrestrial predators either.

The Modern Debate

Significant differences have arisen between the two suborders since their divergence
causing some confusion as to whether they are related at all. There have been numerous
studies using biochemical, molecular, and/or morphological data to analyse the relationship
between megabats, microbats and other taxa.
The 'bat monophyly hypothesis' states these two groups are each
others' closest relatives in an evolutionary sense (i.e. they form a clade). In contrast, the
diphyly hypothesis states that megabats and microbats evolved
independently from two different groups of non-flying mammals. It has been suggested
that megabats are more closely related to dermopterans
and primates than to
microbats10,11.
The bat controversy has raised some interesting questions and forced us to question
long-held beliefs. Systematists have learned that
important phylogenetic questions can only be answered with input from a wide variety of
fields - taxonomy is a truly multidisciplinary subject!